Getting the distance from the screen right is a delicate balance. Too close and the camera picks up moire patterns from screen. Too far, and the camera won't be in focus. (Neither of these problems happen with real resistors.)

If you have a retina display, try shrinking the page (command-minus in Mac Safari) once or twice. The higher resolution of the retina display will allow you to move closer to a smaller image without the camera generating moire patterns.